MRI imaging is well-known for medical applications, in which three dimensional (i.e., volumetric) imaging information of a region of a patient's body is acquired and displayed for diagnostic purposes. The MRI information may be acquired using a variety of modalities and a number of different acquisition devices.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems, for example, store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications.
After image reconstruction, the reconstructed image is stored in an MRI image file, which can be stored either locally, or in a Picture Archive Communication System (PACS). MR image files are usually in a vendor-independent format called Digital Imaging and Communications in Medicine (“DICOM”). Using the DICOM format, each MR image file has a header portion and a body portion. The header portion contains information similar to that located in the raw data header as well as information about the specific corresponding imaging slice, e.g. image slice number. The body portion contains the actual image data. Typically, each MR image file contains image data about one imaging slice.
There are a number of parameters that influence the strength of the signal obtained from an MRI scanner, and the appearance of the acquired image. Some of these parameters are controlled by the operator of the scanner, such as the repetition time (“TR”), the echo time (“TE”), and the flip angle α. Other parameters are characteristics of the tissue being studied, such as the relaxation times T1 and T2. In principle, the unambiguous interpretation of an image involves only the observation and determination of the tissue dependent parameters, such as T1 and T2. In practice, however, these parameters are at least partially obscured by differing selections of TR and TE.
Image contrast between tissue components results from differential rates of “relaxation”, i.e., the transition from transverse magnetization back to longitudinal magnetization. T1 and T2 are two different relaxation constants that result in different image contrast highlighting different tissue components. To create a T1-weighted image, magnetization of the tissue is allowed to recover before measuring the MR signal by changing the TR. This image weighting is useful for post-contrast imaging. To create a T2-weighted image, magnetization of the tissue is allowed to decay before measuring the MR signal by changing the TE. The MRI image can be biased toward either T1-weighted or T2-weighted images, and thereby vary the contrast between tissue components (e.g., fat, muscle, and water), by choosing imaging parameters for the MRI scan, resulting in different image acquisition protocols. In T1-weighted MRI images, fat has higher contrast and water has lower contrast. In T2-weighted MRI images, water has lower contrast and fat has higher contrast. In both T1 and T2 weighted MRI images, air and dense bone (no fat) has the lowest contrast.
Commercially available computer-controlled workstations employ a number of common types of displays to communicate MRI information to a reviewer. For example, MRI displays for the study of breast tissue, e.g., to identify the presence and location of cancer lesions, are well-known. Such MRI displays for breast tissue typically display images showing various two-dimensional slices taken through one or both breasts, and provide the reviewer with the ability to scroll through the respective tissue image slices using a common device, such as mouse. This scrolling enables the reviewer to readily view different slices, eventually covering the entire breast region.
A system operator (e.g., a radiologist, technician, or other medical professional) may employ the MRI scanner to acquire volumetric image information of the patient tissue (e.g., a breast) using different MRI parameters to emphasize different physiological information. For example, T2-weighted images may be acquired with one set of acquisition parameters, and would show different information from T1-weighted images acquired with different scanner parameters. In addition, a set of images (e.g., T1-weighted) may be acquired before the administration of a contrast agent, and thereafter for several time periods after the contrast agent has entered the blood stream.
Typical MRI image acquisition protocols include T2-weighted and multi-phase T1-weighted series/sequences. Some MRI image acquisition protocols also include high-resolution T1-weighted sequences for anatomy clinical readout only. Multi-phase T1-weighted sequences can include pre-contrast and post-contrast T1-w sequences and dynamic series. Other MRI image acquisition protocols also include advanced diffusion sequences, which measure the diffusion of water molecules in biological tissues.
The MRI workstation can also be configured to display a selection of all of these image types, arranged in some order on one or more computer screens. This arrangement of these multiple images is called the hanging protocol (“HP”), and is usually set up by the manufacturer according to the preferences of the particular reviewer. These above-described MRI sequences can be included in various HPs.
HPs are essential features of medical imaging workstation software. Hanging protocols closely correlate with work flow and throughput, and can greatly impact the efficiency of clinical review of MRI data/images. Unlike other medical imaging modalities (e.g., mammograms and tomosynthesis), a standardized HP does not exist for MRI, due to the variations in and the complexities of this imaging modality. For instance, there is no common HP for breast MRI.
HPs vary between manufacturers and acquisition systems. Therefore the reviewer must adjust their reading practice when reviewing images acquired by different systems. This reduces the reviewer's reading efficiency, adding to the time required to read each study. Accordingly, there exists a need for hanging protocols that improve reading efficiency and throughput.